1. Field of the Invention
This invention relates to a surface acoustic wave filter (thereinafter to be referred to as an SAW filter).
2. Description of Prior Art
An SAW filter is a small and thin filter utilizing a surface acoustic wave and is in practical use as a key device for mobile communication equipments such as cellular phones. With the recent demands for a miniaturized, high-frequency cellular phone requiring low power consumption for a long-duration of call, there has been a growing demands for higher frequency SAW filters with lower insertion loss and higher power handling capability.
FIG. 22 is a schematic plan view illustrating a longitudinal coupled double mode SAW filter utilizing a conventional primary-third order longitudinal mode. The SAW filter in FIG. 22 shows also an unbalanced input and unbalanced output type SAW filter. On a main surface of a piezoelectric substrate, as shown in FIG. 22, there are provided three IDTs (Interdigital Transducer) 61, 62, 63 closely disposed to each other along a propagation direction of a surface wave, with grating reflectors (hereinafter to be referred to as reflector) 64a, 64b disposed on both sides of these IDTs. The IDTs 61, 62, 63 are structured by comb electrodes 61b, 61c, 62b, 62c, 63b, 63c having a plurality of electrode fingers 61a, 62a, 63a, with each electrode finger inserted into a space of the other comb electrode. A comb electrode 61b of the IDT 61 is connected with an input terminal In and the other comb electrode 61c is grounded. A comb electrode 62b of the IDT 62 and a comb electrode 63b of the IDT 63 are mutually connected with each other and are connected to an output terminal Out. The other comb electrode 62c of the IDT 62 and the other comb electrode 63c of the IDT 63 are mutually connected with each other and are grounded.
The SAW filter shown in FIG. 22 operates as follows as known. A plurality of surface waves excited by the IDTs 61, 62, 63 are trapped between the reflectors 64a and 64b, and a coupling is generated among the IDTs 61, 62, and 63. As a result, two longitudinally coupled resonance modes of primary and third-order are excited strongly through the electrode pattern, and the filter works as a double mode SAW filter utilizing these two modes by giving appropriate terminations. It is known that a pass-band width of the double mode SAW filter is proportional to a frequency difference between the primary-order resonance mode and the third-order resonance mode.
FIG. 23 shows another longitudinal coupled double mode SAW filter with an unbalanced input and balanced output configuration. On a main surface of a piezoelectric substrate, as shown in FIG. 23, there are provided three IDTs 71, 72, 73 closely disposed to each other along a propagation direction of a surface wave, with reflectors 74a, 74b disposed on both sides of these IDTs. The IDTs 71, 72, 73 are structured by comb electrodes 71b, 71c, 72b, 72c, 73b, 73c having a plurality of electrode fingers 71a, 72a, 73a, with each electrode finger inserted into a space of the other comb electrode. A comb electrode 71b of the IDT 71 and a comb electrode 72b of the IDT 72 are mutually connected with each other and are connected with an input terminal In. The other comb electrode 71c of the IDT 71 and the other comb electrode 72c of the IDT 72 are mutually connected with each other and are grounded. A comb electrode 73c of the IDT 73 is connected with an output terminal Out 1 and the other comb electrode 73b is connected with an output terminal Out 2.
FIGS. 24 and 25 show filtering properties of the longitudinal double mode SAW filter comprising a middle IDT including 27 electrode fingers, side IDTs including 19 electrode fingers each and reflectors including 120 electrode fingers each on a 36-degree Y-cut X-propagation LiTaO3 substrate, and assuming an aperture length of 80 xcexcm and a wavelength xcex of 2.14 xcexcm. FIG. 24 shows properties of the unbalanced input and unbalanced output type SAW filter in FIG. 22. FIG. 25 shows properties of the 50 xcexa9 unbalanced input and 50 xcexa9 balanced output type SAW filter in FIG. 23. A horizontal axis indicates frequency (GHz), takes 1.925 at the middle and is marked in 0.375 GHz increments. A vertical axis indicates an insertion loss and is marked in 10 dB decrements.
Even though the unbalanced input and balanced output type SAW filter can be provided with two output terminals Out, as can be seen from FIGS. 24 and 25, stop-band attenuation of the SAW filter is lower than that of an unbalanced input and unbalanced output type SAW.
The present invention was made to solve the above problem and has an objective to provide an unbalanced input and balanced output type SAW filter capable of obtaining stop-band attenuation approximately equivalent to that of an unbalanced input and unbalanced output type SAW filter.
Also the present invention has an objective to provide an SAW filter which can be fabricated readily when the width between an input IDT and an output IDT is less than xcex/2.
This invention was made to provide a longitudinal coupled multiple mode SAW filter comprising a plurality of IDTs disposed along a propagation direction of a surface wave on a piezoelectric substrate. In the SAW filter, an IDT is flanked on either side by IDTs, one of comb electrodes of IDT in the middle is connected with an input terminal, other comb electrode of the middle IDT is grounded, one of comb electrodes of IDT on each side of the middle IDT is connected with an output terminal, other comb electrodes of the side IDT are grounded. Electrode fingers are so arranged that adjacent electrode fingers of the middle IDT and the IDT on one side makes a connection between terminals or a connection between grounds and the other adjacent electrode fingers of the middle IDT and the IDT on the other side makes a connection between a terminal and a ground.
The SAW filter can be so structured that the output terminals are formed facing one direction and the grounds are also established facing one direction.
Also the SAW filter may be so structured that the output terminals are mutually formed in the opposite directions.
With the above mentioned structure, an unbalanced input and balanced output SAW filter can improve stop-band attenuation.
Also, the SAW filter of the present invention comprises an resonator, which includes one or a plurality of IDTs for exciting and receiving a surface acoustic wave and has antiresonant frequency approximately equivalent to cut-off frequency on the high-pass side of the longitudinal coupled multiple mode SAW filter. The resonator is connected with the input terminal of the SAW filter in series and is provided with an input terminal at the IDT.
The longitudinal coupled multiple mode SAW filter of the present invention comprises two reflectors on both sides of a plurality of IDTs along the propagation direction of the surface wave. These reflectors may reflect the surface wave and trap oscillation energy of the surface wave in between.
The resonator is a one-port resonator and may comprise two reflectors on both sides of one or a plurality of IDTs along the propagation direction of the surface wave. These reflectors may reflect the surface acoustic wave.
When alternating voltage is applied between the input terminal and the output terminal, stress is created between adjacent electrode fingers of the one-port SAW resonator on the piezoelectric substrate by the piezoelectric effect and the surface acoustic wave is excited. By reflecting the exciting surface acoustic wave between two reflectors, the oscillation energy of the wave is trapped between the reflectors, as a result standing wave is generated. The one-port resonator has antiresonant frequency. By connecting the longitudinal coupled multiple mode SAW filter and the one-port resonator in series and matching the cut-off frequency on the high-pass side of the longitudinal coupled multiple mode SAW filter with the antiresonant frequency of the one-port resonator, the roll-off in vicinity of the cut-off frequency on the high-pass side can be made steeper.
In the longitudinal coupled multiple mode SAW filter of the present invention, the width of adjacent electrode fingers of the input IDT and the output IDT may be designed to be smaller than the width of the other electrode fingers.
When the SAW filter operates higher frequency, the space between the input IDT and the output IDT should be designed to be less than xcex/2. Consequently a space between the adjacent electrode fingers of the IDTs becomes narrow, resulting in difficulties in fabrication. In this case, the certain width between the input IDT and the output IDT can be left by designing the width of the adjacent electrode fingers of the input IDT and the output IDT to be smaller than the width of the other electrode fingers, resulting in easy manufacture.
In the longitudinal coupled multiple mode SAW filter of the present invention, the width of adjacent electrode fingers of the input IDT and the output IDT is designed to be smaller than the width of the other electrode and the pitch between one of the adjacent electrode fingers is designed to be narrower than the pitch between the other electrode fingers. Moreover, the width of a plurality of the electrode fingers of the input IDT and the output IDT may be designed to be smaller than the width of the other electrode fingers.
With the above mentioned structure, a certain width between the input IDT and the output IDT can be left even if the frequency becomes further higher, resulting in easy manufacture.
This invention was made to provide a longitudinal coupled multiple mode SAW filter comprising a plurality of IDTs disposed along a propagation direction of a surface wave on a piezoelectric substrate. In the SAW filter, an IDT is flanked on either side by IDTs, one of comb electrodes of the middle IDT is connected with an input terminal, one of comb electrodes of the IDT on each side of the middle IDT is connected with an output terminal, the width of adjacent electrode fingers of the input IDT and the output IDT is designed to be smaller than the width of the other electrode and the pitch between adjacent electrode fingers is designed to be narrower than the pitch between the other electrode fingers.
This invention was made to provide a longitudinal coupled multiple mode SAW filter comprising a plurality of IDTs disposed along a propagation direction of a surface wave on a piezoelectric substrate. In the SAW filter, an IDT is flanked on either side by IDTs, one of comb electrodes of the middle IDT is connected with an input terminal, one of comb electrodes of the IDT on each side of the middle IDT is connected with an output terminal, the width of adjacent electrode fingers of the input IDT and the output IDT is designed to be smaller than the width of the other electrode and the pitch between adjacent electrode fingers is designed to be narrower than the pitch between the other electrode fingers. Moreover, the width of a plurality of the electrode fingers of the input IDT and the output IDT may be designed to be smaller than the width of the other electrode fingers